Capacitive pressure sensors use the property of capacitance to measure pressure. A capacitor is comprised of two isolated parallel conductive surfaces that are separated by a non-conductive dielectric material. Changes in the distance between the electrically charged conductive surfaces in turn change the capacitance.
Capacitive sensors incorporated in pressure mapping systems allow measurement of interface pressure between two surfaces. A pressure mapping system typically incorporates a matrix of capacitors in which thin, electrically isolated conductors are separated by a compressible, dielectric material. When pressure is applied to the capacitor, the distance between the conductors is reduced, resulting in a change in capacitance. The detected capacitance may be correlated to a pressure value. The individual pressure values for each capacitive element are then processed to create a two dimensional map of the pressure distribution. Each cell acts as an ideal plate capacitor and is not subject to change in area or influenced by other capacitors in the array.
Ideally, the two surfaces between which the sensor is placed should be flat or uniform. However, there are often circumstances where it is desirable to measure the pressure distribution on uneven or undulating surfaces. Examples include measuring the interface pressures between a person and their mattress surface, or their seating surface. In clinical environments, this information is used to optimize patient comfort, and ensure that the pressure levels are acceptable over time as to not cause tissue damage or necrosis. The sensor must conform to the two surfaces which it contacts to avoid providing inaccurate data to the pressure mapping system.
Current techniques for producing capacitive sensors involve bonding a thin elastomer to parallel strips of conductive fabric using non-conductive adhesives to hold the conductive strips in position and to isolate them electrically from neighbouring strips. This configuration prevents short circuits and provides geometric stability, as shown in prior art FIG. 1. Two layers of these elastomer/conductor combinations are required, where each intersection of conductors forms a sensel. However, a capacitive sensor formed using this technique has a thickness which adversely affects the suppleness of the final product, reducing the ability of the sensor to conform to surfaces and impacting image quality.
Adhesives are geometrically unstable, deforming upon application of pressure or heat. Upon removal of the pressure or heat, adhesives often create artifacts such as the appearance of pressure after the pressure has been removed. These “ghost” images can contribute to inaccurate pressure values. Adhesives which are less susceptible to deformation result in a lamination that is less pliable when assembled into a sensor. Ideally, the only part of a sensor which should deform is the dielectric which acts as a spring in between the capacitive plates.